US low-cost carrier Southwest Airlines is to ground over 80 Boeing 737 aircraft pending immediate inspections after a fuselage hole was discovered in a jet that depressurised on a service to California.Southwest says that it has "decided to keep a subset of its Boeing 737 fleet out of the flying schedule" to commence an "aggressive inspection effort".It says 81 aircraft are affected by the checks and that these will be examined over the next few days. The jets are covered by US FAA airworthiness directives detailing checks for skin fatigue.Southwest has taken the action after one of its 737-300s, operating flight WN812 between Phoenix and Sacramento, diverted to Yuma yesterday after a loss of cabin pressure and deployment of oxygen masks."Upon landing safely in Yuma the flight crew discovered a hole in the top of the aircraft," says the carrier, adding that it was located about mid-cabin. It has not indicated the size of the rupture.One of the 118 passengers and one of the five cabin crew members were treated for minor injuries.The aircraft involved, identified by the US National Transportation Safety Board as bearing registration N632SW, carries serial number 27707 and is a 15-year old airframe."We have launched personnel to Yuma to begin the investigation process with the NTSB, FAA and appropriate parties to determine the cause of the depressurisation," says Southwest chief operating officer Mike Van de Ven.It says it is working with Boeing on the details of the inspections for the aircraft.Southwest suffered a similar incident two years ago when a Nashville-Baltimore service - also operated by a 737-300 - diverted to Yeager after being holed in its upper fuselage in July 2009.Investigation attributed the hole to fatigue cracks and the incident prompted the FAA to mandate a Boeing service bulletin covering fatigue checks on certain older-variant 737s.

Quoting DLdiamondboy (Reply 16):One must also remember that WN has a very high aircraft utiization of aircarft as compared to other airlines. Which means that these birds are most likely getting close to cycling out of the certificated cycle life. An aircraft fuselage is similiar to a ballon constantly being inflated and deflated introducing metal fatigue. Planes has 39,768 cycles and 48,722 hours so its not new, but far from being close to being out of total service life hours.By comparison the 737-200 from Aloha that had the roof blow off was 19 years old at the time of the accident and had 89,090 cycles

Edit FYI N387SW The SW 733 flight 2294 that experienced rapid decompression in 2009 had Total hours of 50,500 and 42,500 cycles. It was almost the same age as this 733 as well. Just an interesting by point

UMA, Ariz. (AP) — Inspectors have found small, subsurface cracks in three more Southwest Airlines planes that are similar to those suspected of causing a jetliner to lose pressure and make a harrowing emergency landing in Arizona, a federal investigator said Sunday.Southwest said in statement that two of its Boeing 737-300s had cracks and will be evaluated and repaired before they are returned to service. A National Transportation Safety Board member told The Associated Press later Sunday that a third plane had been found with cracks developing.The cracks found in the three planes developed in two lines of riveted joints that run the length of the aircraft.Nineteen other Boeing 737-300 planes inspected using a special test developed by the manufacturer showed no problems and will be returned to service. Checks on nearly 60 other jets are expected to be completed by late Tuesday, the airline said.That means flight cancelations will likely continue until the planes are back in the air. About 600 flights in all were canceled over the weekend after Southwest grounded 79 of its planes.NTSB board member Robert Sumwalt said Boeing was developing a "service bulletin" for all 737-300 models with comparable flight cycle time as the Arizona jet, which was 15 years old and had about 39,000 takeoff and landing cycles.There are 931 such models in service worldwide, 288 of which in the U.S. fleet.Boeing's bulletin would strongly suggest extensive checks of two lines of "lap joints" that run the length of the fuselage. The NTSB has not mandated the checks, but Sumwalt said the FAA is likely to make them mandatory.Friday's flight carrying 118 people rapidly lost cabin pressure after the plane's fuselage ruptured — causing a 5-foot-long tear — just after takeoff from Phoenix.Passengers recalled tense minutes after the hole ruptured overhead with a blast and they fumbled frantically for oxygen masks. Pilots made a controlled descent from 34,400 feet into a southwestern Arizona military base. No one was seriously injured.The tear along a riveted "lap joint" near the roof of the Boeing 737 above the midsection shows evidence of extensive cracking that hadn't been discovered during routine maintenance before the flight — and probably wouldn't have been unless mechanics specifically looked for it — officials said."What we saw with Flight 812 was a new and unknown issue," Mike Van de Ven, Southwest executive vice president and chief operating officer, said. "Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft."Sumwalt said that the rip was a foot wide, and that it started along a joint where two sections of the plane's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear. Further inspection found more cracks in areas that had not torn open.The riveted joints that run the length of the plane were previously not believed to be a fatigue problem and not normally subjected to extensive checks, Sumwalt said."Up to this point only visual inspections were required for 737s of this type because testing and analysis did not indicate that more extensive testing was necessary," Sumwalt said.That will likely change after Friday's incident, he said.The FAA declined to say if it was requiring other operators to check their aircraft for similar flaws.The NTSB also could issue urgent recommendations for inspections on other 737s if investigators decide a problem has been overlooked. The agency's investigation has not determined that the cracks caused the rupture, but it is focused on that area.Federal records show cracks were found and repaired a year ago in the frame of the same Southwest plane.A March 2010 inspection found 10 instances of cracking in the aircraft frame, which is part of the fuselage, and another 11 instances of cracked stringer clips, which help hold the plane's skin on, according to an AP review of FAA records of maintenance problems for the Arizona plane.The records show the cracks were either repaired or the damaged parts replaced. Cracking accounted for a majority of the 28 problem reports filed as a result of that inspection.It's common for fuselage cracks to be found during inspections of aging planes, especially during scheduled heavy-maintenance checks in which planes are taken apart so that inspectors can see into areas not normally visible.The Arizona jetliner had gone through about 39,000 cycles of pressurizing, generally a count of takeoffs and landings. Cracks can develop from the constant cycle of pressurizing for flight, then releasing the pressure.Southwest officials said it had undergone all inspections required by the FAA. They said the plane was given a routine inspection Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.The decompression happened about 18 1/2 minutes after takeoff from Phoenix Sky Harbor International Airport after the pilots reached their cruising altitude. They immediately donned their oxygen masks, declared an emergency and briefly considered returning to Phoenix before the cabin crew told them of the extent of the damage, Sumwalt said."They discussed landing in Phoenix, but quickly upon getting the assessment decided to divert to Yuma because it was the closest suitable airport," he said.The plane's voice and data recorders were being examined in Washington, and Sumwalt said they worked well and showed no sign of a problem before the incident.Southwest operates about 170 of the 737-300s in its fleet of 548 planes, but it replaced the aluminum skin on many of the 300s in recent years, a spokeswoman said. The planes that were grounded over the weekend have not had their skin replaced.Southwest said "based on this incident and the additional findings, we expect further action from Boeing and the FAA for operators of the 737-300 fleet worldwide." Boeing did not immediately return messages left Sunday.US Airways operates 19 of the older-model 737-300s. Airline spokeswoman Liz Landau said they have not been grounded and no additional inspections are being done.In July 2009, a football-sized hole opened up in-flight in the fuselage of another of Southwest's Boeing 737s, depressurizing the cabin. Sumwalt said the two incidents appeared to be unrelated.A fuselage failure, although extremely rare, can have deadly consequences. In 1988, cracks caused part of the roof of an Aloha Airlines Boeing 737 to peel open while the jet flew from Hilo to Honolulu. A flight attendant was sucked out of the plane and plunged to her death, and dozens of passengers were injured.

The US Federal Aviation Administration is readying an emergency airworthiness directive mandating fuselage lap-joint inspection for approximately 175 737 Classic aircraft, in the wake of the 1 April decompression event caused by a fuselage structural failure aboard a Southwest 737-300, forcing it to land in Yuma, Arizona.The FAA says it will require operators of early Boeing 737-300, -400 and -500 aircraft to conduct "initial and repetitive electromagnetic inspections for fatigue damage".The action will "initially apply" to approximately 175 of the roughly 1,800 737 Classics worldwide, though the FAA only has regulatory jurisdiction of 80 US-registered aircraft, most of which the regulator says are operated by Southwest Airlines.

According to Flightglobal's ACAS database, in addition to Southwest, US Airways operates 59 737-300s and -400s and Continental Airlines operates 32 737-500s, though it appears only a fraction of both fleets would be impacted by the airworthiness directive, if at all.US Transportation Secretary Ray LaHood says the 1 April Southwest "incident was very serious and could result in additional action depending on the outcome of the investigation".Southwest says with its knowledge of what the FAA has planned, "we believe the 79 aircraft already identified for inspection will accomplish this directive", for the airline. The carrier adds none of its -500 series aircraft will fall under the airworthiness directive.The FAA emergency airworthiness directive "will require initial inspections using electromagnetic, or eddy-current, technology in specific areas of the aircraft fuselage" on certain -300, -400 and -500 series aircraft that have flown more than 30,000 flight cycles.The FAA adds that repetitive inspections will be required at regular intervals.Boeing has not yet disclosed what distinguishes these 737 Classics from newer models, though a statement by Southwest indicates fatigue life is not the only guide for which aircraft require inspection, with the carrier saying the 79 aircraft under scrutiny were "designed differently in the manufacturing process."As of 22:00 GMT on 4 April, Southwest had inspected and returned to service 64 of 79 Boeing 737-300 aircraft, while three were being held for repairs after additional cracks were found. Those aircraft, says Southwest, will remain out of service until Boeing "recommends an appropriate repair".Further, Boeing is preparing a service bulletin of its own to cover the operators both inside and outside the US.

With the coming of Tuesday's Federal Aviation Administration emergency airworthiness directive, there are still a few key questions left unanswered by the identification of the 175 737-300, -400s and -500s worldwide that will require intensive Eddy-current inspections of their fuselage lap-joints.

For quick catch up, a 737-300 made an emergency landing in Yuma, Arizona on April 1 after developing 5ft hole in the upper fuselage, which has since been traced to a pre-existing structural fatigue.

Only about 80 aircraft in the US are subject to the inspections, and almost all are the Southwest Airlines 737-300s that will all have completed inspection by late Tuesday.

So what of the other approximately 95 Classics around the world?

Boeing says the group of 175 was narrowed down by two criteria:The airframes in question had to have 30,000 or more operational cycles. Southwest says the airframes in question were "designed differently in the manufacturing process". Boeing confirms there are differences in the lap-joint design, and the specific configuration, says the airframer, was phased out as part of a blockpoint change during the 737 Classic's production run. The number of aircraft with this design is significantly higher than the aircraft identified by the FAA and Boeing, though only 175 meet the criteria when paired with 30,000 or more cycles.

The specifics of that design configuration are yet undisclosed, though just how much information is shared publicly is up to Boeing and the FAA.

The natural question that will come along with these available facts is what prompted the different lap-joint design in the first place? And what's being done to ensure the aircraft with this older design along and fewer than 30,000 cycles are properly cared for just as the higher-cycle aircraft?

Paul Richter, Boeing's chief project engineer for the 737 Classics, just held a media conference call about the emergency service bulletin mandating new inspections, and he said that prior to the Southwest Airlines April 1 incident, Boeing engineers thought the lap joints wouldn't require inspections until 60,000 cycles.

He also said the 500-hour repetitive inspections could change after NTSB's investigation finds the root cause of the problem. The 500-hour inspection is "an initial step" that should conservatively address the issue, at least in the short term.

Richter fielded questions eloquently, and with depth, and expressed confidence about the eddy-current inspection technology available today, when queried about whether electromagnetic inspections can adequatly find cracks due to fatigue.

US National Transportation Safety Board (NTSB) member Robert Sumwalt states the lap joint on the Southwest Boeing 737-300 that suffered a decompression event on 1 April was not believed to be an area that would fail on the aircraft.The aircraft, N632SW, made an emergency landing in Yuma, Arizona after a hole opened in the top of the fuselage.Southwest initiated inspections of 79 Boeing 737-300s and subsequently the FAA said it was readying an emergency airworthiness directive covering roughly 175 737-300/400/500 models to conduct initial and repetitive electromagnetic testing for fatigue damage.The NTSB investigating the incident onsite in Yuma concluded their was clear evidence the skin on the aircraft separated at the lower rivet line, and preliminary examinations showed pre-existing fatigue along the entire fracture surface.During a briefing in Yuma, Sumwalt stated the FAA, the manufacturer and the industry "have not believed that this particular lap joint on this model aircraft was one that warranted attention on an aircraft that had this amount of takeoff and landings".There were additional cracks found on the incident aircraft, says Sumwalt, who states the cracks are about a quarter of an inch long.Boeing is expected to release a service bulletin outlining dual frequency Eddy-current inspections, which entail a small probe moving across the aircraft skin, says Sumwalt.He explains he has no specifics of the Boeing service bulletin, but anticipates it would require inspection of both lap joints running the entire length of the fuselage.Sumwalt reiterates that previously it was unknown that the lap joints on an aircraft "of this number of cycles" should be inspected. The assumption, he says, was that 737-300s at 40,000 cycles did not need to undergo non-destructive testing.Flightlglobal's ACAS database shows that N632SW had logged roughly 29,980 cycles.Southwest states it has completed inspections on all 79 aircraft 737-300s and five remain out of service after minor subsurface cracking was found. The aircraft will remain grounded until "Boeing recommends appropriate repairs and those repairs have been completed".

The SB requires inspection of the lower row of fasteners in the lap-joint, along the left and right hand side of the crown of the aircraft at stringer four between Station 360 and 908, making the area under scrutiny about 50ft long. This SB applies to line numbers 2553 through 3132, which were delivered between 1993 and 2000, which had a frame tear strap spacing of 20 inches. Of the 579 aircraft produced, only about 175 have 30,000 cycles or more.Inspections must take place within 5 days on a portion of the 175 that have 35,000 cycles or more.Inspections will be required within 20 days of the remaining balance of the 175 aircraft with between 30,000 and 34,999 cycles.Around 80 of the 175 are in the US, with the majority flying with Southwest Airlines.Inspections will be repeated every 500 cycles until a more permanent solution can be identified as a result of the NTSB investigation.Line numbers 292 through 2552 had a different lap joint design, which Boeing says included a frame tear strap spacing of 10in. That design already fell into a lap-joint modification program that came in to effect for those aircraft over 50,000 cycles.For line numbers 2553 through 3132, Boeing said it anticipated cracks in this area of the fuselage, but not until 60,000 cycles or more.

C'est quoi ce "frame tear strap spacing"

Ca s'arrête en 2000...

Pour mémoire les NG datent de

The first NG to roll out was a -700, on December 8, 1996. This aircraft, the 2,843rd 737 built, first flew on February 9, 1997. The prototype -800 rolled out on June 30, 1997 and first flew on July 31, 1997. The smallest of the new variants, the -600s, is the same size as the -500. It was the last in this series to launch, in December 1997. First flying January 22, 1998, it was given certification on August 18, 1998.[52][55] A flight test program was operated by 10 aircraft; 3 -600s, 4 -700s, and 3 -800s

The National Transportation Safety Board displays the 5-foot-long fuselage skin section that split open on a Southwest Airlines Boeing 737-300 to the media on April 5, 2011, in Washington, D.C. (JEWEL SAMAD/AFP/Getty Images)Boeing engineers knew about potential cracking in the area of older 737 aircraft where a hole opened on a Southwest jet in flight Friday, but thought problems wouldn’t show up until much later, a company official said Tuesday.

“We did anticipate that there would be a need to inspect the airplane at some point in time,” Paul Richter, Boeing chief project engineer for out-of-production aircraft, said in a conference call with reporters.

“However, all of the analysis and testing that supported that activity convinced us that we would not have an issue with this lap joint lower row until much much later in the life of the airplane,” he added. “Our plan previous to this event was to recommend inspections starting at 60,000 cycles, which we felt was a very conservative number, and obviously none of the airplanes of this configuration are anywhere close to that at this point in time.”

The Southwest 737-300 that developed a 5-foot-long hole soon after takeoff Friday from Phoenix had about 40,000 cycles (takeoffs and landings). A lap joint is where two pieces of the fuselage overlap and are riveted together.

Boeing improved the lap joints on these aircraft in response to problems on previous models that resulted in the company calling for replacing the joints on those older aircraft at 50,000 cycles, Richter said. That requirement applies to more than 2,000 older 737s.

Boeing’s current Next-Generation 737 production models, the 747-600, -700, -800 and -900, have several differences in the design of the lap joints, most notably improvements that significantly reduce bending that contributes to fatigue cracks, Richter said. “We remain completely confident that there is no lower-row cracking issue with the lap joints on the NG design.”

Late Monday, Boeing issued a service bulletin calling on airlines to inspect Boeing 737-300s, -400s and -500s with more than 35,000 cycles within five days and with 30,000 to 35,0000 cycles within 20 days, Richter said. He said the inspections take about eight hours per aircraft.

The inspections cover about a 50-foot section of lap joints on both sides of the crown of the aircraft and use eddy-current devices, which “can look through layers of aluminum for disruptions in the magnetic field, which are signatures for cracks that are in the base metal below,” Richter said. “Our inspection techniques today are quite sensitive at finding very short cracks that are at such a length that there’s really no risk of multiple cracks connecting with each other, which is what apparently happened with this event airplane.”

The bulletin mirrors a directive the Federal Aviation Administration put out Monday and applies to about 175 aircraft built between 1993 and 2000. There are about 395 more of these models that have not yet reached 30,000 cycles, Richter said.

The trigger threshold of 30,000 cycles is a “conservative estimate of when cracks might be detectable in this location,” Richter said.

The FAA has called for repeat inspections after 500 cycles as a precautionary step. Richter said Boeing engineers would come up with a recommendation on what inspection interval would be needed as they analyze the incident.

Southwest Airlines completed inspecting all but two of its 79 affected 737s as of Tuesday morning, reporting: “Minor subsurface cracking was found in five aircraft that will remain out of service until Boeing recommends appropriate repairs and those repairs have been completed.”

Boeing has given Southwest repair guidance on three aircraft, calling for replacement of an 18-inch length of lap joint to ensure removal of any undetected cracks, Richter said, adding that the repair should take eight to 16 hours. Boeing will give repair guidance on a case-by-case basis for now and include repair information in a future service bulletin update, he said.

The B737NGs have the Fuselage Tear straps below each Frame unlike a spacing of 20 inches as on the Classics.Also they are wider than the ones on the classic.How effective is this mod.regdsMEL

L'article de flight

Airlines that operate Boeing 737s face a rigorous new emergency inspection regime following the fuselage rupture of a Southwest Airlines 737-300 over Arizona on 1 April. The new requirement will see each aircraft undergo an 8h inspection every 500 cycles.Within three days of the incident, Boeing issued a service bulletin covering the operators of about 175 737-300/400/500s, requiring inspections of fuselage structure to begin within five days for aircraft with more than 35,000 cycles, and within 20 days for those with 30,000 to 34,999 cycles.Meanwhile, the Federal Aviation Administration has issued an emergency airworthiness directive requiring repeated examinations every 500 cycles.The inspection will search for sub-surface cracks in the lap-joint of a subset of 737 Classic aircraft. The cracks have been found to develop far earlier in the aircraft's service life than had been anticipated, says Boeing's 737 Classic chief project engineer Paul Richter.The US airframer issued the service bulletin late on 4 April, requiring dial frequency Eddy-current inspections on the lower row of fasteners in the fuselage lap-joint, along the left- and right-hand side of the crown of the aircraft at stringer four between Station 360 - just aft of the forward passenger door - and Station 908 - a few frames ahead of the rear passenger door. This covers about 15.2m (50ft) - almost the entire length of the passenger cabin.The service bulletin applies only to 737 Classic aircraft with line numbers 2553 and 3132 inclusive, which were delivered between 1993 and the end of the Classic's production run in 2000. Paired with the 30,000-cycle requirement, a total of 175 aircraft worldwide meet this criterion, with 80 operated within the USA, almost exclusively by Southwest Airlines. Most of the others operate in Europe and Asia, says Boeing.What distinguishes this tranche of 579 737s is a design modification that was intended to eliminate the requirement for a lap-joint modification programme already in effect for line numbers 292 to 2552 inclusive, which were required above 50,000 cycles.Richter, who is chief project engineer for Boeing's out-of-production aircraft, including 737 Classics, says Boeing had anticipated some level of structural cracking in the relevant area, but had expected it to occur around 60,000 cycles, far later than the 39,871 cycles of the aircraft involved in the 1 April incident.The design change increased the spacing of the tear strap frames from 250mm to 500mm (10in to 20in) inside the fuselage, which are intended to prevent fuselage damage from propagating across structural frames in the event of a failure. Ultimately, the updated design on the newer 737 Classics was intended to increase the fatigue life of the lower row of the lap-joint.Richter says the service bulletin "currently does not have a repetitive inspections listed in it", but the FAA mandates a 500-cycle repeat interval under its emergency airworthiness directive as a precautionary measure. Richter says the requirement will be reviewed "through the course of our analysis for adjustments as required down the road".He says 500 cycles is a "rare interval to impose - it is quite frequent and has been used before with similar concerns". He adds that Boeing and the National Transportation Safety Board have suggested a precautionary and conservative interval until the root cause of the 1 April structural failure is established.The examinations, which will look for disruptions in a magnetic field, indicating signatures for cracks in the base metal, will take about 8h per airframe with two mechanics in a maintenance environment, taking a further 8-16h to repair any cracks, says Richter.Following the 1 April incident and emergency landing at Yuma, Southwest removed 79 other 737-300s from service and conducted Eddy-current structural inspections on them. Apart from the event aircraft, five other 737s have been removed from service after cracks were discovered in the lap-joints.Richter says Boeing has not yet issued guidance to airlines on how to repair the lap-joints if they are found to be cracking, pointing out that later revisions of the service bulletin will address this, along with any preventive measures.He says Boeing is "completely confident" that no such lower-row cracking issue exists with the lap-joints on any 737 Next Generation models that have been in service since 1997.In November 2010, the FAA required airframers to set life limits for almost 4,200 transport aircraft, giving them between 18 and 60 months to determine how many cycles or hours an aircraft type can safely accumulate.FAA administrator Randy Babbit said in a 6 April US congressional testimony that the FAA would take a more active role in incorporating the Southwest accident into its rule-making. "I want to make absolutely certain that what we learn from this accident gets incorporated into our requirements for reviewing ageing aircraft," he said."I am asking my team to review our ageing aircraft programme to ensure we are asking the right questions and taking full advantage of all available data."

Now that the FAA issued an emergency AD to address fatigue cracking in some 175 Boeing 737 Classics, the question arises: how could have Boeing so wildly miscalculated the interval at which inspections of this particular area of fuselage should occur? It might takes months before anyone knows for sure, but it seems clear that the April 1 incident in which a five-foot gash opened in a Southwest Airlines 737-300 stands to heighten concerns about the scores of narrowbody and regional jets that have become the workhorses of the air transport industry.

Of course, engineers expect high-cycle operating environments to place extra strain on airplane skins due to repeated pressurization and depressurization. However, the incident appears to have surprised Boeing.

Previously, its inspection guidelines called for such checks at 60,000 cycles. After the Southwest 737 developed the tear in its fuselage, Boeing cut that interval in half because the 15-year-old airplane had accumulated only 39,781 flight cycles and 48,740 flight hours.

The AD calls for eddy current inspections of lap joints every 500 cycles on some 175 Boeing 737-300s, -400s and -500s. That will undoubtedly place strain on operators of the type, most of which fly outside the U.S. The requirements called for inspection within five days of receipt of the AD for airplanes that have accumulated 35,000 cycles or more, and within 20 days for airplanes that have flown between 30,000 and 35,000 cycles.

Now, Boeing recommends inspections at 30,000 cycles for a subset of airplanes manufactured between 1993 and 2000, at least until it finishes its analysis of the risks involved. The inspections apply only to models with a specific lap joint design no longer used in production, Boeing Commercial Airplanes vice president of marketing Randy Tinseth wrote in his blog, Randy’s Journal.

As other airplanes with the same design approach 30,000 life cycles, they’ll also undergo the same inspections, he added. Boeing has drawn praise for its prompt response to the incident from Southwest Airlines, and one might even credit the manufacturer for its candor in admitting it could not explain the reason for the fuselage failure in such a relatively young airplane.

But for a company that for years has prided itself on its reputation for engineering excellence, a miscalculation of the magnitude evident in this case might well remove a bit of swagger from Boeing’s famously confident stride

Since there are many lap joints in a single airliner, "if you tried to eddy current every lap joint on every plane, the amount of time and energy it would take would be literally staggering," Cox said.

The FAA is also required by law to weigh the cost of the safety procedures it tells airlines to follow against the possible safety benefits. If there is no evidence of a previous safety problem, it becomes more difficult to justify the cost.

"I don't think the public understands the bar that has to be cleared by regulatory agencies," said Bill Voss, president of the Flight Safety Foundation in Alexandria, Va., an aviation industry-funding organization that promotes safety.

Investigators said the tear along a lap joint on the Southwest jet came despite a recent inspection where the plane was taken apart so that inspectors could see into places not normally visible.

They found 21 instances of cracked frames, which are part of the fuselage, or cracked stringer clips, which help hold pieces of aircraft skin together.

Earlier, NTSB investigators said they had found both cracks that were big enough to see with the naked eye but were hidden between overlapping pieces of metal. They said they had also found subsurface cracks.

That's a significant number and should have been a warning to Southwest's maintenance team that there might be more extensive cracking that wasn't detectable by a visible inspection alone, Voss said.

The plane "hasn't just had a lot of cycles, it has had cycles that have put some wear and tear on the structure," Voss said.

Voss compared the inspection process to removing a tumor. Surgeons should "always check around to see if there are any nodes inflamed or whatever else around it. Depending upon what they find, they can go a step further," Voss said.

Hersman noted that at 15 years, the Southwest plane had only reached what is considered midlife for an airliner.

"If we think something needs to be done, whether the aircraft is 15 years old or 50 years old, we will address it," she said.

etailed inspections of a Southwest Airlines jet that experienced a mid-flight fuselage rupture on April 1, 2011 have revealed possible manufacturing flaws and further evidence of fatigue cracks.

A National Transportation Safety Board (NTSB) preliminary report released on Monday raised new questions about the process for bonding aluminum fuselage skin and how wear and tear may affect certain older model Boeing 737-300 series aircraft.

An examination by safety board investigators of the plane and a cutaway of the damaged skin showed microscopic cracks extending from at least 42 of 58 rivet holes connected to the rupture. Cracks also extended in an area forward of the hole.

A separate inspection of intact rivets showed imperfections in the location and size of several rivet holes, but the NTSB does not know if the problem is related to wear, manufacturing, or another cause.

Additionally, evidence of Southwest’s blue livery paint was found inside a joint where the upper and lower fuselage skin meet and where microscopic cracks had been painted over.

Boeing said in a statement that it could not speculate on what the NTSB’s preliminary findings might suggest about the root cause of the incident involving the 15-year-old aircraft.

Southwest said in a statement that the initial findings “were another step in this ongoing investigation” and pledged cooperation with investigators and regulators “in an effort to determine the cause of events.”

There were nearly six hundred 737-300, 400, 500 series planes made between 1993-2000 with fuselage assembly at the company’s Wichita, Kansas, facility. Boeing changed its 737 skin bonding techniques early on based on analysis and other factors including a similar incident in 1988 involving an Aloha Airlines 737-200. Boeing said it is unclear whether the change contributed to the Southwest incident.

Boeing said it was working closely with the NTSB and any attempt to draw conclusions about the Southwest incident “would be premature and speculative.